The two asteroids missed the Earth by just 248,000 and 79,000 kilometres, a near hit in astronomical terms. Although the largest Near-Earth Objects are constantly surveyed by the NASA Advisory Council Ad-Hoc Task Force on Planetary Defense, the time between detection and impact may be too short to do much good. The two objects that passed the Earth on Wednesday 8 September were only detected three days before. Despite being small (6 to 14 metres diameter) they could have done substantial damage.

"Some time in the next decade, a US president will probably be presented with this dilemma: is it worth spending US$1 billion to deflect a space rock that may never hit Earth?" says Reich.

See also: Close-Shave Asteroid Caught on Camera at Wired Science.